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Feb. 14, 2011 -- Older adults who experience hearing loss may be at increased risk of developing dementia, including Alzheimer’s disease. And the risk of dementia likely increases as hearing loss worsens.

Those are among the major findings of a new study aimed at addressing possible solutions to the exploding problem of dementia, which is estimated will afflict 100 million people worldwide by the year 2050.

The researchers say that interventions that could delay the onset of dementia by even one year could lead to a more than 10% decrease in its prevalence over the next 40 years.

But intensive research is needed to identify what these interventions could be, because currently, scientists have clues and conjectures but little hard evidence of possibly helpful interventions.

“Epidemiologic approaches have focused on the identification of putative risk factors that could be targeted for prevention, based on the assumption that dementia is easier to prevent than to reverse,” the researchers write. “Candidate factors include low involvement in leisure activities and social interactions, sedentary state, diabetes mellitus, and hypertension.”

Hearing loss is one reason for low involvement in leisure and social activities, as well as for a sedentary lifestyle.

Testing for Hearing Problems

Researchers led by Frank R. Lin, MD, PhD, of Johns Hopkins Medical Institutions, studied 639 people between the ages of 36 and 90 who did not have dementia when the research project started.

The participants underwent cognitive and hearing testing between 1990 and 1994 and were followed through May 31, 2008, for the development of dementia, including Alzheimer’s disease.

Of the 639 participants, 125 had mild hearing loss, 53 had moderate hearing loss, and six had severe hearing loss.

After almost 12 years, 58 of the participants were diagnosed with dementia, including 37 with Alzheimer’s disease.

Researchers say the risk of dementia was increased among those with at least a mild 25-decibel hearing loss and went up as hearing problems worsened. In other words, those with the most severe hearing loss were most likely to be diagnosed with dementia, including Alzheimer’s.

For participants aged 60 and older, 36.4% of the risk of dementia was associated with hearing loss, according to the study. The risk of specifically developing Alzheimer’s increased with hearing; the more severe the hearing loss, the more the risk. For every 10 decibels of hearing loss, the extra risk increased by 20%.

Researchers found no association between self-reported use of hearing aids and a reduction in risk of dementia or Alzheimer’s.

Investigating the Hearing Loss-Dementia Link

“A number of mechanisms may be theoretically implicated in the observed association between hearing loss and incident dementia,” the researchers write.

It’s possible, they write, that dementia may be overdiagnosed in people with hearing loss, or that those with cognitive impairment may be overdiagnosed with hearing loss.

The conditions may share a common, underlying cause. The researchers write that hearing loss may be “causally related to dementia, possibly through exhaustion of cognitive reserve, social isolation” or other factors.

If their findings are confirmed in further research, their study “could have substantial implications for individuals and public health,” the researchers write.

“Hearing loss in older adults may be preventable and can be addressed with current technology” such as digital hearing aids and cochlear implants, the researchers say. Also, other rehabilitative interventions might be developed that would focus on “optimizing social and environmental conditions for hearing,” the researchers write.

“With the increasing number of people with hearing loss, research into the mechanistic pathways linking hearing loss with dementia and the potential of rehabilitative strategies to moderate this association are critically needed,” the researchers write.

They say hearing loss is associated with dementia, but whether it is a marker for early-stage dementia or a risk factor for dementia that could be modified remains to be determined.

The study is published in the February 2011 edition of Archives of Neurology.